The present invention generally relates to an apparatus comprising an integral diffuser and deswirler for a centrifugal compressor and a method for fabricating and assembling the parts comprising the integral diffuser and deswirler.
The centrifugal compressor is an apparatus typically used to increase the flow of the incoming air to the combustion chamber of a gas turbine engine. These centrifugal compressors are often comprised of an impeller for accelerating an incoming, axially directed airflow to increase its kinetic energy; a radially vaned diffuser surrounding the impeller to decrease the velocity of the airflow emerging about the circumference of the impeller and thereby increase its static pressure; a path having a 90° bend to redirect the radial airflow once again into an axially oriented direction; and a plurality of axially arranged deswirler vanes to reduce turbulence in the axially oriented airflow by converting the high tangential velocity component of the airflow exiting the diffuser to a more useful static pressure. Such centrifugal compressors are often used in aircraft auxiliary power units (APU), since they are relatively light, compact, and highly efficient for their weight.
In order to illustrate the issues that arise in fabricating a centrifugal compressor, FIG. 1 is provided, showing a typical centrifugal compressor 100 illustrative of the prior art. An upper portion of the centrifugal compressor 100 is shown as being symmetrical about a compressor centerline 130, with the lower portion omitted. The modules both forward and aft of the static portions of the centrifugal compressor are similarly omitted for ease of illustration. An impeller turbine 120, to which a plurality of impeller vanes 110 are attached, rotates about an axis of rotation coincident with the compressor centerline 130 to axially draw an airflow 140 through an inlet of a bleed port 150. The axially oriented airflow 140 is accelerated and compressed by rotation of the impeller turbine 120 to that it is expelled in a radial direction along the outer edges of the impeller vanes 110. The airflow 141 emerging from between the impeller vanes 110 may be directed radially through a passage having a forward wall defined by a shroud 151, a forward diffuser wall 152, and an outer deswirler wall 153, the passage also having an aft wall defined by an impeller back shroud 154, an aft diffuser wall 155, and an inner deswirler wall 156, the wall being formed by the assembly. The airflow 141 is directed by the passage through a plurality of diffuser vanes 160 where the velocity of the airflow 143 is reduced prior to entering into the combustion chamber 145. A 90° bend 170 redirects the airflow 143 again into an axial direction where it passes through a plurality of deswirler vanes 180 that remove centrifugal motion from the airflow 143 and direct the airflow 144 to a combustion chamber 145. The edges of the forward and aft diffuser walls 152, 155 may be mated to the outer and inner deswirler walls 153, 156 at mating points 192, 194, respectively. It can be seen that these mating points 192, 194 may provide discontinuities in the outer and inner passage walls, respectively. These discontinuities may allow leak paths to develop as the different parts thermally expand and contract. Leak paths may also develop if the parts become warped, misaligned, or flexed by applying compressive force through fastener 190 and fastener 195 when the assembly is attached to the gas turbine engine. Furthermore, improper or loose assembly can cause fretting and wearing that can reduce engine performance, adversely affect reliability, and lead to premature failure.
The diffuser and deswirler components of the typical centrifugal compressor are generally constructed as a separate module that is attached to the gas turbine engine by one or more fasteners. This module may be constructed by assembling a number of components, each of which may be fabricated using standard sheet metal techniques well known to the industry. These components may be interconnected by using standard fabrication methods well known in the art. For example, the edges of two such components may be held together by providing the edges with duct tails, bayonet fittings, simply supported edges abutting one another, or clamped ends, to name several common fabrication methods. Such fabrication methods may make the module cumbersome and difficult to assemble and align according to required tolerances for efficient operation. They may also require many additional parts, such as numerous fasteners; require additional features to be incorporated into the component that are unrelated to its function, e.g. bayonet fittings; be expensive to manufacture; and be prone to fretting, wearing, and other undesirable actions caused by vibration of the parts against one another during operation of the gas turbine engine. Furthermore, utilizing multiple components for the fabrication of the module may often result in a discontinuous flow path along the junctions of the components, particularly along the inner passage walls, which can result in turbulence and undesirable leak paths, both of which may have a negative effect on engine performance. Finally, the close tolerances and clearances between these components, which enhance the performance of the gas turbine in general, may be negated because of limitations in manufacturing methods of the components and by the cumulative build up of these tolerances.
The diffuser and deswirler components may also be constructed as a single module by casting the diffuser and deswirler components in a single, monolithic structure. However, casting may require a thicker structure, which in turn may result in increased weight of the module. Such increased weight is undesirable in an aircraft assembly. Also, complex shapes may be difficult to achieve by casting and may require multiple casting, careful alignment, and further machining, all of which increase labor cost. Usually some combination of casting and sheet metal fabrication is used, but not always. U.S. Pat. No. 4,854,126, to Chevis, discloses a diffuser system fabricated using a one-piece casting having the diffuser vanes and the deswirler vanes integral with the casting. The assembly has an annular central hub which enables the cast housing to be slid into the turbine casing about the shaft of the turbine engine for positioning at the end of the engine. The one-piece casting must be machined to appropriate finished surface tolerances after casting, which adds to the time and cost for assembly. Installation of the casting requires removal of the impeller disk and insertion of the turbine shaft through the bearing surfaces of the annular central hub. An intake structure is then bolted to the engine casing over the casting to precisely mate with the diffuser and deswirler vanes to form airflow passages therethrough.
In today's competitive APU engine market, a low weight, high performance, and low cost design is often desirable. As can be seen, there is a need for an integrated assembly to perform the diffuser and deswirler functions in a centrifugal compressor, where the integrated assembly is lightweight, not prone to turbulence and leak paths along its airflow path, easy to construct and fabricate. The assembly of such an integrated assembly should not promote the accumulation of close tolerances that would detract from the efficiency of the module.